When boomers were kids in the 1950s, 60 percent of U.S. families consisted of two married parents: a breadwinner and a homemaker. Today only 20 percent of American children live in such a family. Instead, couples divorce — or never marry in the first place — and form new households, raising their kids in a tumble of step- and half-siblings. And although the divorce rate has been declining among younger couples, among boomers it has increased 50 percent in the past 20 years — with no slowdown in sight. "The baby boomers are likely to have the highest lifetime levels of divorce of any generation born in the 20th century," predicts Andrew Cherlin, professor of sociology and public policy at Johns Hopkins University in Baltimore.

But behind that unhappy statistic lies an idealistic impulse. If boomer families are more fluid than others, it's because that generation has heightened expectations of family life, says Stephanie Coontz, author of Marriage: A History. Raised during a postwar period of ever-increasing prosperity, boomers saw their families' fortunes improve steadily, and they accepted that as the natural state of affairs — not only for their own financial future, but for life in general. "We expected things to get better every year," Coontz says. "We had a willingness to go for more in our family lives and a willingness to leave if things didn't work out."

Optimistic boomers rewrote the rules for families. They fought for a woman's right to pursue rewarding work outside the home — and a man's right to be a full partner in parenting. And while a great many boomers embraced traditional values, the generation as a whole turned divorce and cohabitation from taboo to commonplace. Boomers also demanded respect for all kinds of familial bonds, including interracial and gay partnerships, single parenthood and interracial adoption. As a result, the very definition of family in this country has changed. These days, unlike in previous generations, a single parent with a child is called a family; an unmarried couple with a child is called a family; a same-sex couple with a child is called a family. "Our concept of families is more flexible today," says Susan L. Brown, codirector of the National Center for Family & Marriage Research in Bowling Green, Ohio. "It's not 'Here's a rigid, narrow definition, and now you have to conform.' It accommodates whomever we view to be part of our families."

Among all these changes, though, one thing remains constant: Americans' love for their kin. In survey after survey, an overwhelming majority of people say that their family is at the center of their lives. As boomer Michael J. Fox once put it, "Family is not an important thing. It's everything."

Single Mom to Stepmom

The rate of single parenthood in the U.S. has increased dramatically since 1970, as Americans marry later in life, or not at all. And divorce among boomers is also common and rising. When these trends collided in a New England home, a new blended family was born.

Modern Factor: "I didn't need to find Mr. Wonderful so I could start a family," says Catherine Anderson, a 46-year-old middle school teacher. "I created a family when I was economically stable and emotionally ready."

Anderson, far right, adopted her son Sam, fourth from right, in 2005 and gave birth to his brother Marcel, second from left, via a sperm donor in 2007.

Then, in 2011, she met her fiancé, David Beseda, second from right. Beseda, 61, is a divorced social worker and the father of five grown children, including, from left, Rachael, 31; Hannah, 25; Luke, 29; and Jason, 30. (Not pictured is Hannah's twin, Emma.) All told, David "has a hundred years of parenting experience," says Anderson. "I really get to capitalize on that."

Vanessa Hua and husband, Marc Puich, relax with their twins and Hua's mother, Sylvia, on the lawn of their shared home in Orlinda, California. — Gregg Segal (Groomer: Tacha Scott)

Living With Grandma

Modern Factor: Last year, writer Vanessa Hua; her husband, Marc Puich; and their 2-year-old twins — Tobias, far left, and Luka — moved in with her widowed mother, Sylvia, 75. For Vanessa, it was a return to the Bay Area home of her youth.

"I think it's very special to be able to raise my kids in the place where I was a kid," says Vanessa, 39. "You get to think about your childhood and relive it, in a sense."

Having multiple generations share a household is a custom in China, where Sylvia Hua and her late husband were born. But the choice is getting more common in the U.S., because of high housing costs and the growing influence of immigrant communities. Vanessa blogs about her household at threeunderone.blogspot.com.

The close quarters can lead to the occasional tiff, but nothing that can't be talked through, says Vanessa. She adds that Marc, 41, who works in software, is "very easygoing."

Grandmother Sylvia, who still works as a food-safety scientist, says she loves living with the twins: "They like to spend time in my room, because I have lots of things there that they want to learn about. Life is more enriched and fun in this arrangement."

Into the Mix

Some states banned interracial marriage as late as 1967, when the Supreme Court ruled such laws unconstitutional. Today 15 percent of all new marriages are mixed race.

Modern Factor: When Paula Windham married her husband, Thomas, in 1966, their interracial union was unusual. "Back then it was unheard of," says Paula, 71, who is Chicana. (Thomas, 70, is African American.) "Fortunately, I had a fairly supportive family," Paula continues. "Once they knew what I wanted, they got used to the idea."

The Windhams met in college, and Thomas later became a clinical psychologist. Now semiretired, the couple live in Boulder, Colorado, where they raised three children. Their two sons — Carlos, 42, and Yusef, 39 — live on opposite coasts, but daughter Khadija Rennix, 42, lives a short drive away with her husband, Will, 39; their daughters, Camille, 6, and Anaya, 2; and Khadija's daughter Alicia, 12, whose dad is Khadija's former partner, Eric Lind, 40.

As interracial relationships have grown more common, racial categories have blurred, Paula notes: "We are a mixed-race couple, and our children are also in mixed-race relationships. I think the fact that we've been able to deal with that as a family has led to the longevity of our marriage. It has also given our children a sense that they don't have to check a specific box when they are asked what they are."

Making It Legal

Massachusetts became the first state to recognize same-sex marriage, in 2003. Now 17 states and the District of Columbia allow gay couples to wed, and many longtime partners have taken the plunge.

Modern Factor: "I always hoped that gay marriage would come to fruition, but for so long it seemed like something from a novel from outer space," says Leona Strong, 65, who has been married to Grace Sonya Harper, 62, since 2008. The two women have been together for nearly 27 years.

Leona, a retired airline ticketing manager, was previously married to a man and has three biological kids: sons George, far left, and Kennaz, far right, and daughter Kimberly, in the patterned top. Leona's ex-husband, she says, is very much a part of the family: "There's no hardship or any type of uneasiness among us. We are very fortunate in having everybody get along."

Though feeling "gay since birth," Leona says it wasn't until she nearly died during a medical procedure in 1975 that she chose to follow her "true life." At the time, her children were all under age 10. "I told them, 'I'm still going to be your mom,' " she says. " 'No matter what they say about us, we are going to remain a family.' " When Grace, a municipal safety official, entered the picture, she quickly became part of that family. Now there are two grandchildren, Brandon (in red) and Justez (in orange), and two great-grandchildren, Taryn (on George's lap) and Alysha (center). They call Leona "Grancie" (third from left) and Grace "Grandma Sonya."(next to Strong). (Also pictured above is goddaughter Lynnette, in the blue shirt.)

"I have a wonderful wife," Leona says. "We have a beautiful home. We have a great family. We're the two happiest old people you could ever meet." Massachusetts became the first state to recognize same-sex marriage, in 2003. Now 17 states and the District of Columbia allow gay couples to wed, and many longtime partners have taken the plunge.

Party of Two

Autism has grown more common in this country, and boomer parents tend to keep their autistic adult children at home with them.

Modern Factor: "I want everything for my son that any mother would want," Shebah Carfagna says of 24-year-old Geno. "The only difference is, he probably won't leave me."

Geno has a form of autism that would make it difficult for him to live independently. "Other people have grown children in college or off making decisions on their own, and I don't have that option," says Shebah, 57. "But I tell people it's not a sad story. It's a great story."

Six years ago, Shebah left a career in fundraising and public relations to launch a personal-training company, Panache Fitness. As her own boss, she has more flexible hours to meet Geno's needs. (She is separated from Geno's father, who sees his son on the weekends.)

"Geno gave me the courage to become an entrepreneur," Shebah says. "I love what I do."

Her son's days are busy with classes, a job bagging groceries, and sports. "He's a social butterfly," Shebah says. "I work to make sure he's not just in the community, but a productive member of the community."

Boomerang Bunch

The average age of first marriage keeps rising: It's now 26.6 for women, 29 for men. Many young adults return home after graduation and before launching their careers and new families.

Modern Factor: Linda and Mark Michele have four children: one in high school, one in college, one out on her own, and Sean, 24, far left, who returned home after grad school last year in the face of a tough job market.

"He got his MBA, and he really wants to be an entrepreneur," says Mark, 55. Sean's parents are happy that he can stay at home while working part time and launching his own company. "He's very respectful and is not just sitting around playing video games," Mark says. "He does not hesitate to do the dishes or mow the lawn."

A diminished job market may play a role in many such cases, but another factor is the relative harmony between the generations. Unlike in the era of the generation gap, lots of millennials genuinely want to live with their boomer parents — and vice versa.

The Micheles bond through sports. Both Mark and Linda, 53, coach (he, basketball; she, lacrosse) at New York's East Rochester High School, where he is athletic director and she teaches physical education. Sean's company, SPM-Sports, organizes girls' lacrosse tournaments. Daughter Whitney, 27, far right, is an assistant women's lacrosse coach for the University of Massachusetts. Jordan, 20, second from left, plays lacrosse, while the youngest, Casey, 15, plays multiple sports.

But it's not all ESPN all the time with this crew. "We have just as much fun watching The Bachelor on TV," Mark says.